1. Field of the Invention
The present invention relates to communication systems, and, in particular, to calibration of related clocks employed for separate modes of operation in a communication system.
2. Description of the Related Art
Many types of communication systems, especially communication systems employed for information transfer with wireless (including cellular) devices, employ two or more modes of operation depending on the relative processing activity of a given device. One example of switching between multiple modes of operation is the switching between an active mode and an inactive mode, such as the commonly termed “sleep” mode, in a wireless or cellular communication system. During sleep mode, portions of a device's circuitry are disabled, such as by disabling the power or clock signals to the device's processing hardware. Switching a device to sleep mode when the device is not required to send or receive information allows the device to, for example, conserve power or generate less interference noise for other devices in the system.
In wireless communication systems, a handset or other type of mobile device (collectively termed herein as “user equipment” or “UE”) generally switches to a slow clock during sleep mode that is used to track system time and periodically “wake up” the device. During a wake-up period (or wake-up mode), the UE transitions to active mode and typically carries out activities such as re-synchronizing to serving-cell timing, checking for incoming calls, and adjusting power control. Once these activities are completed and the UE is not required to communicate with the network, the UE returns to sleep mode.
For a UE to perform tasks during wake-up mode, the UE receives data from its associated network. In Wireless Code Division Multiple Access (WCDMA) communication systems (such as those operating in accordance with a version of the UMTS WCDMA or IMT-2000 standard), data is transmitted between a network cell (e.g., base station) and a UE as encoded sequences, which are received by synchronizing the UE receiver to the exact sequence timing, which is the frequency and code phase of the sequence employed to encode the basic data. Timing of sequences within a cell is termed “cell timing” and is determined with relatively high resolution. During wake-up mode and fully active mode for network communication, a fast clock (relative to the slow clock of sleep mode) is employed by the UE, which, for WCDMA systems, is termed a global counter (GC) upon which the cell timing is defined. Due to characteristics of mobile communications, the cell timing of the received data may fluctuate in time, and, consequently, the UE receiver dynamically adjusts its GC (fast clock) in response to fluctuations of the cell timing.
When the UE transitions to wake-up mode, the GC is set to proper timing according to the actual time elapsed over the last sleep interval. To set the timing of the GC, a calibration procedure is performed in which the time interval related to the slow clock is converted to a corresponding time interval related to the fast clock. Without errors introduced by the system, and a known ratio R between the frequencies of the fast and slow clocks, the conversion from the slow clock time interval to the fast clock time interval is computed by multiplying the slower clock interval by R.
However, in practice, the ratio R fluctuates over time due to temperature gradients and electrical noise. Temperature changes tend to introduce drift in clock frequency, while additive electrical noise tends to introduce duty-cycle fluctuation in the clock signal. Consequently, in practice, the ratio R should be dynamically measured and then used to set the fast clock frequency/phase for the GC when the UE transitions to wake-up mode. This procedure, termed clock calibration, starts when the UE transitions to wake-up mode and includes a clock ratio measurement process and a fast clock setup process. During the clock ratio measurement process, the ratio R is determined by i) defining an interval of N counts of the slow clock (also called a calibration time interval or measurement time interval), ii) measuring the corresponding number M of counts of the fast clock during the calibration interval, and iii) setting the ratio R as M/N. During the fast clock setup process, the number L of counts of the slow clock during the previous sleep interval is measured, and the number T of counts of the fast clock that would have registered had the fast clock been kept on during that previous sleep interval is calculated as T=(M*L)/N, where (L/N) is defined as the ratio between the sleep time interval and the calibration time interval.
Due to added noise and temperature variations, errors may arise in measuring the number M of counts of the fast clock during the calibration interval. When M is multiplied by L/N, which may be relatively large, the errors of M may be magnified resulting in relatively large errors in fast clock count T. These relatively large errors in T can cause the UE to lose synchronization with the cell. As an example, suppose that a system UE operating in accordance with a WCDMA standard employs a slow clock of 32,768 Hz and a fast clock of 61.44 MHz, resulting in a ratio R of 1875. Given a measurement time interval N of 64 msec and a sleep time interval L of 1.024 sec, the ratio L/N is 16. Lab measurements using real UE hardware have found that the error in M may be as large as ±25 counts. When calculating T, this error is magnified by 16 times (i.e., L/N) such that the resulting error in T is ±400 counts. For a WCDMA system, where the fast clock rate is 16 times the base chip rate of the WCDMA system (e.g., 3.84 MHz), magnification of an error in the slow clock can result in an offset of 25 chips, causing severe degradation of the WCDMA system's synchronization performance.